About thirty years ago one of the inventors, K. Larsson, in Z. phys. Chem. 56 (1967), 173, reported that the aqueous system of monocaproin, which is a lipid the complete name of which is glycerolmonohexanoate or glycerolmonocapronate, forms one single liquid phase at all compositions. At that time this was a unique and remarkable behaviour of a single amphiphilic (surfactant) molecule; only mixtures of surfactants and cosurfactants were known to exceptionally show such lack of a phase transition when the composition was changed from anhydrous towards pure water.
Recent work within the field of drug delivery, with special regard to molecules that need to be protected against enzymatic degradation (like peptides), has shown that so called microemulsions or L2-phases provide a useful carrier system. An example of such an L2-system is disclosed in European patent specification No. 314 689, which discloses the utilization of C.sub.16-22 -monoglycerides interacting with C.sub.16-22 -triglycerides and a polar liquid.
although the phase properties of monocaproin have been known for a very long time, however, it has not been realized earlier that this unusual micellar phase can provide a highly efficient carrier in connection with for instance drug delivery, as far as is known to applicant. The main reason probably is that a molecule with such a short hydrocarbon chain is regarded more as an organic solvent than as a lipid and has, therefore, not been expected to provide efficient solubilization power of drugs into an association-colloid type of structure.
When amphiphilic systems forming ordinary micellar solutions (L1-solution) have been used in drug delivery, relatively long hydrocarbon chains have also been involved; cf. K. A. Johnson, G. B. Westermann-Clark, and D. Shaf, Pharmaceut. Res. 6 (1989), 239.
Furthermore, it can be added that Ericsson and Hegg in Progr. Colloid & Polymer Sci. 70 (1985), 92, have reported a study of the surfactant behaviour of 1-monocaproin and its interaction with ovalbumin in a diluted water solution. The critical micellar concentration (cmc) of monocaproin was found to be 160 mM. Their result demonstrated that there is no molecular interaction between this specific protein and monocaproin. We have now unexpectedly found that the micellar system according to the present invention exhibits full compatibility with any protein also at high concentrations, i.e. even up to and including the region where the L1-type of structure changes to the L2-type of structure.